The present invention relates to an exposure controller, more particularly to an exposure controller for a camera incorporating an electronic iris.
Instead of mechanical exposure control, electronic exposure control has recently been used to meet the needs for miniaturization of electronic cameras.
Conventional electronic exposure control has been disclosed in Japanese Laid-open Patent Application No. 5-48975, for example.
A conventional exposure controller is described below. FIG. 8 is a block diagram of this conventional exposure controller. Referring to FIG. 8, numeral 81 represents a rectifier circuit for receiving an image signal from an image pickup device at an input terminal and for rectifying the image signal, numerals 82 and 83 represent comparator circuits, numerals 84 and 85 represent reference voltage setting circuits, numeral 86 represents a counter circuit, and numeral 87 represents an exposure time control circuit for controlling exposure time.
The operation of the exposure controller having the above-mentioned configuration is described below. First, an image signal from the image pickup device is rectified by the rectifier circuit 81. A reference voltage, that is, a potential difference corresponding to the amount of change in the output level of the image signal when the amount of input light entering the image pickup device is doubled, is generated by the reference voltage setting circuit 84. Another reference voltage, that is, a potential difference not less than the above-mentioned potential difference, is generated by the reference voltage setting circuit 85. The two reference voltages generated by the reference voltage setting circuits 84, 85 are compared with the output level of the rectified image signal by the comparator circuits 82, 83, respectively. The count of the counter circuit 86 is incremented, decremented or stopped depending on the outputs of the comparator circuits 82, 83, and the exposure time of the image pickup device is controlled by the exposure time control circuit 87 depending on the output of the counter circuit 86.
However, in the above-mentioned conventional configuration, the level of the picked-up image signal at each exposure time control process changes larger as the exposure time becomes shorter, and the problem of hunting occurs at the convergent point of the image signal level.
Accordingly, an object of the present invention is to provide an exposure controller free from hunting by using a coring value adapted to the discharge pulse count of a solid-state image pickup device.